TEXTILE MATERIALS AS BARRIERS AGAINST ELECTROMAGNETIC RADIATION

The accelerated development of textile products that have shielding properties against electromagnetic (EM) radiation excites the interest of scientists, the textile and clothing industry in the manufacture of woven fabrics, knitted fabrics and clothing with shielding properties. This paper deals with the investigation of the shielding effect of the electroconductive fabric to be used for manufacturing protective clothing consisting of textile and non-textile components. The textile component is a cotton-modacrylic blend, and the non-textile component is an inox yarn inserted into the fabric every 1 cm in the transverse direction of the fabric. The fabric was finished using solvents in the processes of dry and wet cleaning as potential care processes. The measurement results of shielding fabric properties have shown that the degree of shielding is better preserved after 10 cycles of wet cleaning than after dry cleaning.


INTRODUCTION
Field and waves of wireless technologies (GSM or global system for mobile communications, the internet, etc.) are nonionizing microwave electromagnetic (EM) radiations [1]. The development and use of mobile phones and various electronic devices raises the interest of scientists in different areas. Intensive development of new technologies and products, e.g. electrical and electronic devices, implies the need to control the negative impact and the possibilities of their prevention. Various medical research activities have shown that the frequent use of electronic and electrical devices can affect an increase in stress, insomnia, headaches, heart arrhythmias as well as an increase in the percentage of cancer diseases, behaviour changes etc. [2 -6].
Electromagnetic radiation hazard is thus higher, since negative consequences are noticed only after a long period of time, and are usually not associated with EM pollution but with other factors (poor nutrition, accelerating pace of life, stress and so on). Therefore, the EM emission limits of all electrical and electronic devices are set so as to minimize the possibility of interference with radio and wireless communications. When EM rays pass through the medium or material [5], they interact with material molecules, and this interaction phenomenon can be divided into three parts: • absorption, • reflection, • secondary reflection.
When EM rays hit the material surface, they push the charge in the material to oscillate. This forced charge oscillation behaves as an antenna and results in reflection. The second part is converted into heat energy due to oscillation. This kind of signal loss is known as weakening due to absorption. Thus, the shielding property of the material against electromagnetic radiation is based on the reflection from the conductive surface and the absorption in the conductive volume. Part of the wave is reflected, while the rest is transmitted and weakened while passing through the medium, Figure 1. properties [5] The shielding effect of the conductive barrier (dB) is the sum of reflection losses (R), absorption loss (A) and secondary reflection loss (Rr). The combined effect of these losses (reflection and absorption) determines the effectiveness of shielding properties of the material which depend on: • electric and magnetic properties, • conductivity properties on the surface and in the inside, • material thickness, • material composition.
With respect to all of these hazards and risks that occur due to exposure to sources of electromagnetic radiation there is a growing need to find ways of shielding against radiation. Among the various solutions offered, the attention of researchers has been attracted by textile products and composite textile-based materials due to their diverse application and conformity [7 -11].
Conductive textile structures can be designed in various ways. Fabrics with a different degree of electroconductivity can be designed from yarns made of filament or cut fibers in combination with traditional non-conductive fibers. Another way of producing an electro conductive structure is achieved by coating textile substrates with conductive substances, which do not impair the fundamental substrate properties [12]. It is possible to achieve satisfactory conductivity of fabric through the incorporation of conductive particles into the fibers, with at least 15 % of conductive particles [12].
Studies were conducted in which the influence of material type, metal presence in the material, number of threads, weaving density, number of coats and number of layers was investigated. Shielding properties of fabric against EM radiation increase with the increasing number of layers, with decreasing yarn fineness and with the presence of metals in the fabric [13 -15].
The paper deals with the influence of solvents on shielding properties of electroconductive modacryl/cotton blend fabric with inox threads. Its application area is the manufacture of work clothes for employees at petrol stations, power lines, gas works, and other power plants where they need protection due to a high intensity of the electromagnetic field. Considering that this fabric is intended for making work wear, the influence of the care process on the durability of protection is very important. Examination of the degree of fabric shielding with inox yarn as a barrier/shielding against electromagnetic radiation was carried out at frequencies: 0.9 GHz, 1.8 GHz, 2.1 GHz and 2.4 GHz before and after the first, third, fifth, seventh and tenth cycle of treatment with solvents perchloroethylene in the dry cleaning and water in the wet cleaning process.

EXPERIMENTAL
The investigation was carried out on samples of modacryl/cotton blend fabrics with inox yarn whose construction features are listed in Table 1. The dark blue shielding satin fabric, in which the inox yarn is inserted in weft direction with repeat 1 cm, was woven at Čateks Company, Čakovec, Figure 2.

Fabric care
The electroconductive fabric was tested in the dry cleaning process (DC) through 10 cycles. This process implies perchloroethylene (PERC) treatment with an addition of cleaning booster whose purpose is to colloidally dissolve small quantities of water in PERC having low surface tension and dipole moment. PERC molecules do not aggregate, have greater molecular mobility than water, so they penetrate into the textile material faster and take away dissolved dirt, which speeds up the process. Fastness test of the fabric to PERC in the dry cleaning process was done according to standard HRN EN ISO 1375-2:2010 [16].
Wet cleaning (WC) is a professional cleaning process in water, with the help of special machines, programs and ecological detergents, which allows the cleaning of extremely sensitive clothing intended for hand-washing and dry cleaning. The role of the special detergent is to remove fine dirt and grease from the textile material and then to keep them in the bath in order not to return to the textile which could diminish the cleaning effect [17,18]. Fastness test of the material to wet cleaning was done according to HRN EN ISO 3175-4 2010 [19].

Measurement of the effectiveness of electromagnetic shielding
The effect of the care process on the shielding properties of the fabric was investigated using the method developed at the University of Zagreb, Faculty of Electrical Engineering and Computer Sciences, at the Microwave laboratory, Department of Radiocommunications, under working conditions: • temperature 23 ± 1 °C, • relative humidity 50 ± 10 %.
EM shielding factor was determined as the ratio between the EM field intensity (E 0 ) measured without the tested fabric and the EM field intensity (E 1 ) with the fabric placed between the radiation source and the measuring device.
Shielding effect, SE (dB) is calculated according to expression (1): where: E o -field level without shielding, E 1 -field level with shielding.
According to the recommendations of international standards IEE-STD 299-97 [20], MIL STD 285 [21] and ASTM D-4935-89 [22], a measuring system was designed which consists of the measuring instrument NARDA SRM 3000, signal generator, horn antenna and 1 m x 1 m wooden frame shielding into which samples are placed, Figure 3. Spectrum analyzers are instruments that measure the strength of the measured field from a particular frequency using frequency sweep over a wider frequency bandwidth and have the ability to process and store results. NARDA SRM 3000 measuring instrument is a portable spectrum analyser that measures the field isotropically, i.e. from any direction and any polarization, making measurements more convenient and simpler. It is intended for EM field measurements in the frequency range between 80 MHz and 3000 MHz, where it provides an extremely linear response [23].
The signal generator is an instrument that serves as a source of electromagnetic radiation. Sometimes it has to be used together with a microwave amplifier to obtain higher values of the electromagnetic field. For the purposes of this work a generator of continuous sinusoidal signal at frequencies of 0.9 GHz, 1.8 GHz, 2.1 GHz and 2.4 GHz was used.
Horn antenna was named after its shape and is used for receptions and transmissions of microwave signals. During reception it serves to collect and direct radio waves to the waveguide, while during transmission it serves to direct radio waves from the waveguide to the space.
The wooden frame was designed for the purpose of testing the effectiveness of shielding against electromagnetic radiation.
During measurement the wooden frame with a sample is inserted into the wooden stand and fixed to remain in vertical position.

RESULTS AND DISCUSSION
The results of the effectiveness of the electroconductive fabric before and after 10 cycles of the care process in weft and warp direction as shielding against electromagnetic radiation were obtained after measuring at frequencies of 0.9 GHz, 1.8 GHz, 2.1 GHz and 2.4 GHz, Tables 2 and 3.    Figure 4 shows the values of the effectiveness of shielding against EM radiation (SE) of the untreated samples with inox fibers in weft direction before and after 1, 3, 5, 7 and 10 dry cleaning cycles.
It can be seen that the functional fabric with inox yarn has a high degree of shielding at 1.  The values of the effectiveness of SE functional fabrics as shielding against electromagnetic radiation in warp direction before and after 1, 3, 5, 7 and 10 dry cleaning processes are listed in Table 3.  SE values of fabric as shielding against electromagnetic radiation in weft direction before and after 1, 3, 5, 7 and 10 wet cleaning cycles are listed in Table 4.  The SE of the samples listed in Table 4 indicate the continuous reduction of shielding properties of the functional fabric after wet cleaning cycles. The reduction of SE values of the fabric after 10 wet cleaning cycles measured at all frequencies is somewhat lower in comparison with SE values after 10 dry cleaning cycles. Figure 6 shows the comparison of the untreated fabric samples with inox yarn in weft direction before and after wet cleaning measured at all tested frequencies whereby it can be observed that the lowest SE values are at a frequency of 0.9 GHz. The SE values of the fabric in warp direction before and after 1, 3, 5, 7 and 10 wet cleaning cycles as shielding against electromagnetic radiation are listed in Table 5.

CONCLUSION
The paper presents the investigation of functional conductive fabric which consists of two textile components: modacryl/cotton and non-textile component: inox yarn which is every 1 cm inserted only in weft direction (transverse threads in the fabric). Measurements of the SE values of the textile shielding before and after 10 treatment cycles in dry and wet cleaning were taken in warp and weft direction. Shielding effect (SE) testing of the electroconductive fabric before and after treatment in solvents, perchloroethylene and water, as shielding against electromagnetic radiation was carried out at frequencies of 0.9 GHz, 1.8 GHz, 2.1 GHz and 2.4 GHz. Shielding properties of the fabric in weft direction are satisfactory at frequencies of 1.8 GHz, 2.1 GHz and 2.4 GHz, while they are extremely poor in warp direction.
The SE values of the functional electroconductive fabric in weft direction after 10 dry and wet cleaning cycles at frequencies of 0.9 GHz, 2.1 GHz and 2.4 GHz confirmed that the wet cleaning process is more acceptable for the care of this shielding fabric than the dry cleaning process. An additional contribution of the wet cleaning process is manifested in a better environmental profile compared with dry cleaning.